"They" is most often used as third-person plural pronoun. However, they can also be used to refer to a single something without a specific gender (as can the possessive, objective, and reflective forms of they: their, them, and themselves).
When you use they to refer to a single something, it's called the "singular they".
Here's an example of how it works:
- A movie star can have a big influence on their fans.
In the above example, "movie star" doesn't have a specific gender, so using the possessive form of "singular they" works because gender doesn't affect the sentiment.
Many people also use the "singular they" if they are gender non-conforming, non-binary, or simply don't identify as either a man or a woman. In any of those instances, "singular they" (and its forms) are used to refer to a single person.